Oil country tubular goods (OCTG) used in oil fields and natural gas fields can have an overall length which sometimes reaches several thousand meters. Such long oil country tubular goods are typically assembled by connecting a great number of steel pipes having a unit length of about ten to twelve meters by means of couplings which are short tubular members. The material of which the steel pipes and couplings are formed is usually carbon steel, stainless steel, or high alloy steel, depending on the environment of use.
These steel pipes are connected by threaded engagement of a pin having a male threaded portion on its outer peripheral surface and a box having a female threaded portion on its inner peripheral surface. Typically, a pin is formed on each end of a steel pipe, and a box is formed on the interior of a coupling. The connecting part formed by a pin and a box is a threaded joint.
In the case of a threaded joint for steel pipes for which a high degree of airtightness is required, the tip of the male threaded portion of the pin and the base portion of the female threaded portion of the box each have an unthreaded metal contact portion formed thereon. By inserting one end of a steel pipe into a coupling and tightening the male threaded portion of the pin and the female threaded portion of the box until the unthreaded metal contact portions of the pin and the box are made to contact each other, a metal-to-metal seal which ensures airtightness is formed.
FIG. 9 schematically illustrates a threaded joint having this type of pin-box structure. A pin 1 has a male threaded portion 1a on its outer peripheral surface and an unthreaded metal contact portion 1b on its tip. Correspondingly, a box 2 has a female threaded portion 2a on its inner peripheral surface and an unthreaded metal contact portion 2b inwards thereof. Makeup, i.e., connection of OCTG is performed by engaging the threaded portions of the pin 1 and the box 2 with each other and tightening the threaded portions until the unthreaded metal contact portions of the pin 1 and the box 2 contact each other.
In order to carry out periodic inspection and the like, OCTG are lifted out of a well, and breakout is performed, i.e., the threaded portions of a threaded joint are loosened and the steel pipes are separated from the coupling. After the completion of inspection, makeup of the OCTG is performed again by tightening the threaded portions of the pin and the box, and they are again lowered into a well. The sliding contact surfaces of the threaded portions 1a and 2a of the pin 1 and the box 2 and the unthreaded metal contact portions 1b and 2b repeatedly experience a strong frictional force at the time of makeup and breakout of OCTG. Accordingly, if a threaded joint does not have sufficient resistance to frictional forces, incomplete sealing (poor resistance to leakage) and galling (unrepairable severe seizure) occur in the threaded portions 1a and 2a and particularly in the unthreaded metal contact portions 1b and 2b when tightening and loosening are repeatedly carried out.
Therefore, threaded joints for steel pipes for use in makeup of OCTG are required to be able to (a) resist tensile forces in the axial direction due to the weight of connected steel pipes, (b) resist pressure from internal and external fluids, and (c) maintain good leakage resistance and galling resistance when they are repeatedly used (undergo repeated makeup and breakout) at least four times in the case of casing (large diameter pipes) and at least ten times in the case of tubing (small diameter pipes). In recent years, there has been a trend for the depth of oil wells to become deeper and deeper, and the frequency of use of OCTG in severe conditions such as in polar regions is increasing, so increasingly strict demands are being made with respect to the quality of threaded joints.
In the past, as proposed in JP H01-12995B, for example, in order to provide the contact surface (including the surface of the threaded portions 1a, 2a and the unthreaded metal contact portions 1b, 2b) of the pin 1 or the box 2 of a threaded joint with improved galling resistance, the surface was subjected to surface treatment such as copper plating or phosphate treatment, and a compound grease (also referred to as dope) containing heavy metals such as Pb was applied to the treated surface for lubrication.
Today, however, in view of the prevention of global environmental pollution which is becoming an urgent problem, the use of dope containing Pb is being restricted. Dope which does not contain heavy metals such as Pb, Zn, and Cu (referred to as green dope) has been developed and is being used, but its lubricating performance is inadequate, and the occurrence of galling cannot be prevented particularly when a threaded joint is made of a material which is relatively susceptible to galling such as stainless steel or high alloy steel.
Other methods of improving leakage resistance and galling resistance which have been proposed include (1) a method in which a fluororesin powder is dispersed in a plating layer, (2) a method in which a lubricating protective coating is formed by sputtering, and (3) a method using a solid lubricating coating instead of compound grease, but none of these methods has achieved sufficient leakage resistance and galling resistance.